Technical Abstract:
Profitability and sustainability of cow/calf operations are dependent on cow efficiency. Annual forage consumption is a logical input component included in cow efficiency models and large cows generally consume more forage annually than small cows. The ratio of additional kg of calf weaning BW to each 100 kg additional mature cow BW could be used as one practical indicator of the efficacy of increasing cow size for the purpose of increasing ranch profitability. While many factors affecting efficiency are either difficult to measure or low in heritability, mature BW is highly heritable and can easily be obtained at the time of weaning. A total of 1111 calves out of Brangus dams and Angus, Bonsmara, Brangus, Charolais, Gelbvieh, Hereford, Maine Anjou, Red Poll and Romosinuano sires were sampled over an 8 year period at the USDA Grazinglands Research Laboratory in El Reno, Oklahoma. Cows grazed abundant native rangeland and were supplemented with hay during inclement winter weather and were provided a protein supplement during winter. Calves were weaned at 202 ± 23 d. Means ± SD for cow age, adjusted cow BW at weaning, cow BCS at weaning, calf birth weight and adjusted calf weight at weaning were 4.84 ± 2.05 yr, 620 ± 67 kg, 5.22 ± .80, 42.6 ± 6.7 kg, 250 ± 33 kg, respectively. The mixed procedure of SAS was used with cow sire breed, cow dam breed, calf sire breed and sex of calf declared as fixed effects and year of birth as a random effect. Cow weight at weaning was adjusted to a constant BCS (5.0) and age (5.0 yr) prior to analysis. An increase in cow BW of 100 kg tended to increase calf weaning BW by 2.34 kg (P = 0.07). Additionally, for every 1 kg increase in calf birth weight, calf weaning BW increased by 2.07 kg (P<0.05). The minimal increase in calf weaning BW associated with increasing cow size on this commercial operation suggests that larger cows may be less efficient. However, the relationship of mature cow BW to calf weaning BW may differ in divergent environments and therefore should be determined on an individual ranch basis.